When you send an email to a Gmail address, Google’s email system will disregard anything you put after a “+”. For example, emails send to firstname.lastname@example.org will be sent to email@example.com. There is no need to create another email account.
This feature is handy for labelling and tagging purpose. For example you can use the email address firstname.lastname@example.org for subscribing to newsletters. You can then label “davidwest+mailing” to easily group and find newsletter emails.
In addition to “+” plus symbol, Gmail also ignore any period in an email address. Thus emails send to “email@example.com” and “firstname.lastname@example.org” will all be sent to “email@example.com”.